1. Field of the Invention
The invention relates to light strings, and particularly to a light string with over-current protection function.
2. Description of Related Art
Light strings are widely used for decorative purposes, especially for holiday lighting. Light strings refer to incandescent filament lamps or LED lamps connected electrically in a series. Generally, a light string includes more than 10 incandescent filament lamps, and is powered by mains electricity, such as 110V AC mains electricity. Each of the light connected in the light string typically includes a lamp, a socket for receiving the lamp, and a holder for receiving the socket and two metal conductive pieces which are electrically connected to the lamp. The two metal conductive pieces are electrically connected to the metal conductive pieces of adjacent lights via electrical wires.
In the conventional light strings, when the lamp fails and the lamp remains in the socket, or when the lamp is removed from its socket for replacement, the closed path for the flow of electrical current is interrupted and the remainder of the lamps in the string will no longer be illuminated. To prevent the occurrence of this condition, a “smart light” is invented. In the smart light, a protruded end made of insulating material is protruded from a lower end of the socket, and the two metal conductive pieces are formed to have elastic structures. When the socket with a lamp is inserted in the holder, the protruded end is inserted between the two metal conductive pieces, thus the two metal conductive pieces are electrically isolated. When the socket with the lamp is removed from the holder, the two metal conductive pieces are electrically connected depending on the elastic structures, thus the remainder lamps can still illuminate.
When the lamp is an incandescent filament lamp, to prevent fails of the light string because of one or more lamps are burned out, the lamps of the smart light use bulbs with a fuse. The fuse will short the two pins of the bulb once the filament of the lamp is burned out thus the remainder lamp can still illuminate. For smart LED lamp, when one or more lamps are not illuminated because of over current, the failed lamp(s) is(are) substantially short circuit, thus the remainder lamp can still illuminate. However, the total resistance of the light string becomes lowered when any lamp is burned out or is removed from the holder, and a current of the light string become larger. When the number of the burned lamps and removed lamps reaches to a certain number, the power of the light string will exceeds the rated power of the reminder lamps, this may cause burned out of all the reminder lamps, even fire.
To prevent this situation, a resistor or a diode or a combination of a resistor and a diode whose resistance is close to that of the lamp is utilized to be connected parallel to each lamp in an existing smart light, thus the fuse in the incandescent filament lamp is omitted. When the lamp is burned out or is removed, the resistor or the diode or the combination of the two is connected in series in the circuit, thus the reminder lamps can still illuminate, and a total resistance of the light string will not lowered, over current will not occur. However, this structure of the light string uses a lot of resistors/diodes, and causes big power loss, large heat release, and high cost. Therefore, an improved light string is needed.